| Abstract: Comets are leftovers thrown out of the inner solar system to the outer regions at the time of the formation of the planets. They spend most of their time far away from the Sun and hence are expected to retain undisturbed primordial material. When they approach the Sun, the surface material begins to sublimate, forming a coma and tail. Spectroscopic, photometric and polarimetric observations of the coma and tail provide great insight into the constituents of the comet and, hence, by proxy, of the material at the time and place of formation. Indian telescopes have been instrumental in advancing our understanding of the properties of dust and the molecular composition of various comet classes: short- and long-period comets, dynamically new and old, compositionally typical and depleted, as well as dust-rich and dust-poor. Classification of comets in terms of their orbital period as short and long-period, is typically on the basis of human timescales. Long-period comets are classified as dynamically new if it can be shown, by tracing back their orbit, that they are approaching the inner solar system for the first time. Based on the molecular emission signatures of the carbon content, comets are classified as either typical or depleted.
It is clear that the process of planet formation is not unique to the Solar system and is a common occurrence across the Galaxy. This is exemplified by the discovery and characterisation of three interstellar objects in the last decade, two of which are comets. Indian telescopes were used very effectively in the observation of the two interstellar comets, although they were much fainter than the more well-observed solar system comets. In this talk, I shall discuss our results on these objects in the context of our work on solar system comets.
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